This was supposed to be the digital election, but it wasn't. Then the TV debates changed everything – except they didn't. What actually happened was the party that spent the most on advertising won the most seats. There's an £18m legal limit on election spend, but with individual candidates allowed to spend an additional £40,000 each, the Tories had a budget of £25m, Labour around £10m and the Lib Dems much less.

In March, the Tories reappointed their trusty old attack dogs, M&C Saatchi, to work alongside the lead agency, Euro RSCG, and M&C Saatchi's chief executive, David Kershaw, wasted no time in setting out his stall, saying: "It's a fallacy that online has replaced offline in terms of media communications."

The Tories used plenty of posters – the party's media buyer, MPG, booked more than 1,000 sites in marginal constituencies – with creative work switching rapidly between attack ads on Gordon Brown to personality pitches for David Cameron. "In that way, it's just like the previous elections," says Mark Mendoza, MPG's chief executive. "The vast majority of the money was spent on outdoor. The Conservative party had a strategy in place for 18 months, and stuck to it. The other parties came to the game very late and suffered all sorts of problems, from media price inflation to creative issues."

Since Maurice Saatchi signed up for Margaret Thatcher's 1979 campaign, political posters have been as much a part of the election debate as the policies and leaders. We look back at the campaigns.

2010 I took billions from pensions. Vote for me M&C Saatchi/Euro RSCG

At the start of the campaign, both parties were mashing up each other's posters online, while the Lib Dems' spoof Labservative party won plaudits. The "airbrushed Dave" ad was ripped to shreds on mydavidcameron.com, as was the "I've never voted Tory before but … " poster. At the end of March, however, M&C introduced their trademark, brutally simple, attack posters and Twitter went quiet. The ads were classic assaults on Labour's performance with sharp, mocking copy, essentially the same as "Labour isn't working" but with one key difference – the posters were changed on a weekly basis with new copy. As Saatchi himself says of political advertising: "It's a world of trial by combat, in which you hit and are hit."

Trevor Beattie's Hello Boys shock tactics came to the fore with a proposed poster depicting Michael Howard as a flying pig, prompting accusations of antisemitism from the Conservatives. The posters were withdrawn and Beattie quit TBWA the day before the poll. Subsequent ads were less provocative, using the slogan "If you value it, vote for it". Iraq was studiously ignored. "The truth is, the Labour majority was so massive and Blair was still very popular so we weren't really too worried," explains Paul Bainsfair, the then chairman of TBWA.

2001 He voted Labour in 1997 / She voted Labour in 1997 TBWA

Beattie said he'd been waiting for the call for 15 years. Under his direction New Labour launched a "job done" campaign ahead of the poll. Alongside attack posters of William Hague with Margaret Thatcher's hairdo, the campaign listed the government's achievements.

1997 Enough is enough BMP DDB Needham

"In most of the 15 general elections held since the end of the second world war, the Conservative party has been the election campaign innovator," says Paul Whiteley, a professor of politics at the University of Essex. "But this began to shift in the 1990s." After years of ad hoc ad teams, the Labour party formally appointed BMP, creating a more coherent strategy. The campaign essentially compared the Tories' 1992 election promises with evidence they'd broken them. "The two-headed John Major had a powerful subtext commenting on Conservative infighting at the time. The overall effect was to make Major look like a Janus," says the TBWA adman Steve Henry.

1992 Labour's tax bombshell / Labour's double whammy M&C Saatchi

Maurice Saatchi, reconciled with the party after Thatcher's departure and now in charge of his new agency, created the tax bombshell poster himself. Midnight Cowboy's director, John Schlesinger, directed a party election broadcast showing John Major returning to his Brixton roots. "In the end, I don't think the ads won it," says Bainsfair. "It was Kinnock's triumphalist election rally in Sheffield that lost it."

1987 Labour's policy on arms Saatchi & Saatchi

The ads targeted Labour's unilateral disarmament policy, and political broadcasts ended with a fluttering union flag, to the strains of I Vow to Thee My Country. "A typical Saatchi political ad from their golden era," says Henry. Sir Tim Bell ran the Tory campaign at Saatchis from 1979. By 1987 he'd left and Thatcher was worried. Poor opinion polls led to her recruiting Bell's new agency, Lowe Bell Communications, to help, although Bell insists: "Maurice was very active in keeping Labour out of power for nearly 20 years."

1983 Like your manifesto, Comrade Saatchi & Saatchi

Tory ads compared identical points in Labour and Communist manifestos, as part of Thatcher's fight against "socialism without and socialism within".

1979 Labour isn't working Saatchi & Saatchi

The poster that changed British political history. Or was it? Tim Delaney was working on the Labour campaign at the time: "The Saatchi poster went up in October 1978 for only two weeks and the election wasn't until May 1979. Professor Ivor Crewe [the then editor of the British Journal of Political Science] published an analysis of voting in 1979 showing the poster had little effect on voting intentions. But it did change political advertising." Labour discovered the queue included Saatchi staff, made a fuss and gave the poster plenty of publicity – a legend was born.